This invention relates in general to attenuator circuits and more specifically to attenuator circuits employing bootstrapping.
Circuit elements such as field effect transistors (FETs) have been used to provide variable impedances in signal attenuators. For example, they have been used in limiters for noise reduction systems where compressors and expanders require controlled limiting of the amplitude of a signal. In U.S. Pat. No. 4,498,055, a field effect transistor is used in conjunction with a capacitor to form a high pass filter with a sliding band type high frequency shelf response whose corner frequency varies as a function of the control signal to the gate of the transistor. In this particular high pass filter circuit the drain-source path of the FET shunts the signal to be limited to ground.
A field effect transistor can be used in conjunction with an inductor to form a sliding band circuit with a low frequency shelf response operating downwardly. A field effect transistor may also be used in conjunction with a resistor to form a fixed band circuit whose frequency range does not change, in contrast to the sliding band attenuators.
When used in noise reduction systems, it is desirable for the circuit elements such as field effect transistors to be operating within a range of signal amplitudes where the impedances of the elements are linear, i.e. does not introduce waveform distortion into the signal. For FETs, this means that the transistors should be operated with their drain-source voltages within a range depending on the type of FETs so that the drain-source current of the FETs vary linearly with the voltage applied across the source and drain. When the drain-source voltage applied to a FET is not within such range, this introduces distortion in the waveform of the signal undergoing compression or expansion of dynamic range. It is therefore desirable to provide attenuator circuits employing elements such as FETs for which the above described difficulties are alleviated.
One way to alleviate the difficulty is to limit the signal amplitude across the element. This, however, reduces the signal to noise ratio and is disadvantageous. It is therefore desirable to provide attenuator circuits which avoid distortion and yet maintain a good signal to noise ratio.